The in vivo administration of the cholecystokinin analogue, caerulein (CER) in excess of the concentration required to induce maximal secretion (hyperstimulation) leads to the rapid formation of acute pancreatitis in mice as defined by pancreatic edema and in-creased serum levels of anylase and lipase. Bombesin (BOM) hyperstimulation, however, causes only an increase in edema. Concomitantly, CER-treatment totally blocks secretagogue- stimulated secretion in vitro, while bombesin induces a milder inhibition. The proposed studies will examine the pathophysiological mechanisms which underlie the alteration in acinar cell function in response to hyperstimulation by CER or bombesin, and the relationship between the diverse cellular actions of these agonists. The nature of the block in acinar cell stimulus-secretion coupling will be investigated by identifying the step(s) of the process which is altered by hyperstimulation. Cholecystokinin, muscarinic cholinergic, and bombesin receptor binding; polyphosphatidyl inositol turnover; cytosolic free calcium changes; secretagogue- stimulated protein kinase activity (calmodulin-activated protein kinase and protein kinase C); phosphorylation of cellular components in response to agonist stimulation; and disruption of acinar cell cytoskeletal components will be examined. Once located, the mechanism by which the block is induced will be investigated. The alterations in pancreatic physiology which lead to tissue disruption and the release of acinar cell products into the interstitium and hence the bloodstream will also be examined. Intrapancreatic activation of trypsinogen to active trypsin will be assessed in response to hyperstimulation by both CER and bombesin. Alterations in acinar cell lysosomal function and the possible activation of trypsinogen by the lysosomal enzyme cathepsin B will be examined. The tissue and subcellular distribution of trypsin will be assayed by immunohistochemical techniques on the light and electron microscopic levels. The temporal and spatial correlation between the secretory and lysosomal compartments in control and treated mouse acinar cells will be examined. The differing actions of CER and BOW hyperstimulation will aid in the identification of the alterations in acinar cell function which are associated with the development of acute pancreatitis. Because of similarities of this model with forms of human pancreatitis, these studies will lead to a better understanding of the factors underlying the predisposition and development of acute pancreatitis in man.